Strain tester



Sept.26, 1939.

E. H. LAND STRAIN TESTER Filed Oct. 1, 19%6 1 v P A TTORNEYS.

Patented Sept. 26, 1939 i 4 h v i UNITED STATES PATENTV oFFcE STRAIN TESTER Edwin H. Land, Wellesley Farms, Mass., assignor, by mesne assignments, to Polaroid Corporation, i Dover, Del., a corporation of Delaware Application October 1, 1936, serial llo. 103,189

5 Claims. (Ci. 88-14) V This invention relates to a new and-improved posed of material available in relatively large apparatus employing polarized light, and more' areas andadaptedfo'r useyin connection with the specifically to an apparatus of the character deaforesaid recentlydeveloped strain-testers. It v scribed for use in the'e xaminat'ion of photoelastic contemplates morevr the provision of such a 5. ff ct r the ik i sensitive tint plate in the form of a resilient sheet 5= Objects of the invention are to provide 'a new with means associated therewith for altering the and improved sensitive 'tint plate or the like for tension or strainplaced upon said sheet when in use in connection with apparatus of the character position between a polarizer and analyzer, wheredescribed; to provide apparatus employing such a by the characterstcs imparted by said sheet to sensitive tint plate for the examination of relathe transmitted beam are' predeterminedly jo tively large objects, if desired without the use of altered. e. lenses and the like; to provide a sensitive tint' In the drawin Figsl and 2 represent a s ai plate the characteristics of which may be altered tester Or he' `embdying e s cture of the in a predetermined manner for use in connection present invention. IO represents any light source;

with apparatus of the character described; to pros for x mpl a nd s nt l m nd H 15 vid such itiv ti t, plate f r iient mat representsany suitable reflector associated thererial, and preferably of a plastic material, such for With. In front of the lamp is 'p a po rexample as a vinyl compound; andto provide r which may mp s p f ly a sheet means associated with apparatus of the character Of l g -p zingmaterial mounted and held described for subjecting such a sensitive tint plate rigidly po t This. l nt' m y omprise 0 to variable strains, whereby its characteristics are D f r bly a diffusing light-polarizer, or it may ate ed have associated with it means for diffusing 'the Other objects of the invention will in part be light emanatvng m the source IO. A suitable obvious and will in part appear hereinafter. Cliffllsing` 1ght-p01a1'iZer y o pr se a sheet of The invention accordingly comprises the fea- -p l i material, such for e mple as 'a 25 tures of Construction, combination of elements, eet of the material made and sold under the and arrangement of parts, which will be exemtrade name "Po1aroid, laminated to the ground plied in the Construction hereinafter set 'forth, face Of &Sheet f ground sia s., Where such a and the scope of the application of which will be plarizer. S p oyed it is' to be. understood that indicated i the claims the glass supporting plate should preferably be 30 For a fuller understanding of the nature and DOSSOned ne rthe light Source n e p l robjects of the invention, reference should be had Zng Sheet, .for if the polarizing el t d its to the following detailed description taken in consupporting P o e p i d c o e y djace t nection with the accompanying drawin wheret so t ain a s m h heating of t h supporting plate may give rise to undesirable 35 Figure 1 is a view in perspective and somewhat p e ast c effects therein. diagrammatic, with enclosing means omitted, of a |3 is represnta Of t e analyzer, which in device embodying o'ne form of the invention; turn may comprise a sheet of light-polarizing ma- Fg 2 is a view in elevation of the dv ce shown terial such as Polaroid, mounted, if desired, on a 40 in Fig. 1 with certain parts removed and other sheet of glass.

parts shown in section: and h It will of course be obvious that any suitable Fig. 3 is a perspective View of a variable sensipolarizing and ana y means may beemp yed, tive tint plate of the type employed in the 'present d t e light o f e S used, may e D invention, together with its mounting and sup.. tioned at any suitable point to illuminate the fitld porting elements. v of View; r h

It has heretofore been customary to employ, in The polarizer is preierably so positioned with connection with polariscopes, strain-testers and respect to the analyzer that the polarizing axes of devices utilizing polarized light for the examinathe two elements are crossed, and these elements i tion of photoelastic effects, sensitive tinteplates. are preferably so formed and positioned that the 0 These plateshave been uniformly expensive and polarizing axis of each element is at an angle of '50 of small surface area, andhave been found un- 45 approxi'mately to the vertical. For example, suited for use with the recently developed strainin Fig.` ltthe polarizing axis of the element |2 testers employing relatively large areas of ligftmay extend at an angle of 45 to the vertical polarizing material. This invention contemplates from the upper right to the lower left portion of the provision of a novel sensitive tint plate comthe element; while the polarizing axis of the ele- 55 ment |3 may extend from the upper left to the lower right portion thereof and at an angle of 45" to the vertical. When so positioned the polarizer and analyzer are crossed, i. e., a minimum amount of light is transmitted, and if the field is viewed, as for example by an observer to the left of the r device shown in Fig. 2, it will appear dark.

The sensitive tint plate ll is preferably positioned closely adjacent the polarizer !2, and between the .polarizer and the analyzer l3. This plate may comprise any light-transmitting, preferably transparent, resilient substance having a high photoelastic coefiicient and preferably showing little or no cold flow. It should comprise preferably such a substance available in relatively large areas in sheet form at relatively low cost.

The preferred material for use as the sensitive tint plate of the present invention is the material now being made and sold commercially under the trade name "VinaL" This material is a vinyl compound, namely, vinyl aceto acetate, plasticized with triglycoldihexoate. It is available in sheet form in any desired area and in any desird thickness. A suitable thickness may be approximately .010 inch. This material when sub'jected to uniform strain, as 'for example by stretchlng, and when interposed between the polarizer and analyzer of a device such as is shown in the draw- This may ing, may function as a full wave plate for a certain predetermined portion of the spectrum, as for example the yellow-green portion, and as more or less of a full wave plate for the remain ing portions of the -spectrum. When subjected to a stretch of a different degree, the material may function as afull wave plate for another predetermined part of the 'spectrum and as more or less than afull wave 'plate for the remainde` thereof. v

Where thedevice is employed in connection with a structure such as is shown in the drawing, the direction of stretch of the sensitive tint plate Il should preferably be at an angle of 45 to the polarizing axes of the polarizer and analyzer. In the device shown in Figs. 1 and 2, for example, the sensitive tint plate may be employed with means adapted to impart to it a stretch in a vertical direction, and the degree of stretch may be controlled so that the properties of the material as 'a sensitive tint plate' may be varied at will. For example, theVinal may be aflixed to rods !5, preferably of glass, either by cementlng the Vinal thereto or otherwise afllxing it thereto. Care should preferably be taken to so mount the Vinal onthe rods that a substantially uniform stretch over its entire surface may be secured.

' be accomplishedby providing a roller IS to which the uppermost rod may, for example, be attached in such a manner that as the roller is rotated each end of the rod is drawn toward it an equal distance. Means not shown may be provided for holding the roller in any desired position, so that the sheet ll may be subjected to any desired stretch.

While Vinal is a preferred material for use in the present invention, it is to be understood that other materials maybe employed. For example, thin sheets 'of rubber have been found satisfactory.

The operation of the device is as follows: The beam of light emanating from the source ID is 'polarized and preferably difiused as it traverses the element !2. The element M, which has been subjected to a predeterminedst'rain, such for example as to impart the conventional sensitive tint to the beam, acts thereon as substantially a,

full wave plate in the yellow-green portions of the spectrum and as more or less than a full wave plate in the blue and'red. The analyzer |3 then functions to block those portions of the beam upon which the sheet M has acted as a full wave plate, and the rest of the beam `is transmitted. In addition to providing a sensitive tint plate of large area, the device may be adjusted until precisely a predetermined retardation is Secured.

When the device is not in operation the tension on the sheet !4 may be released by reversing the direction of rotation of the roller IS and the sheet, which is resilient, will return to its original form. Under such conditions, it may have no effect upon the characteristics of the transmitted beam of polarized light. The preferred material for use, Vinal, shows substantially no flow when cold, and hence when the sheet is subjected to the predetermined desired strain, its properties as a sensitive tint plate remain fixed for a considerable period of time.

Since certain changes may be made in the above construction and different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpretedas illustrative and not in a limiting sense.

It is also to be understood 'that the following claims are intended to cover all of the generic and specific features of.the invention herein described, and all statements of the 'scope 'of the invention which; as a matter of language, might be said to fall therebetween.

*Having described my invention, what I claim as new and useful and desire to secureby Letters Patent is: I

` 1. In a device of the character described, in combination, a polarizer, an analyzer'positioned with its polarizing axis substantially at right angles to the polarizing axis 'of the polarizer, a resilient sensitive tint plate positioned between the analyzer and the polarizer, and means to impart to said plate a substantially uniform stretch in a direction -at an angle 'of substantially 45 to the polarizing axes of said polarizer and said analyzer, said plate comprising a transparent resilient sheet having a high photo'elastic coeflicient and showing substantially no cold flow.

2. A device of the character described comprising light-polarizing means, an analyzer, a resillent sensitive tint plate positioned between said polarizing means and said analyzer and comprising a sheet of a transparent, resilient vinyl` acetal compound adapted when' subjected to a predetermined stress to function as a substantially full wave plate for a portion of the visible spectrum only, means for subjecting said plate to a substantially uniform stretch, and means for altering said stretch in a predetermined and uniform manner.

3. A device for examining photoelastic effects and the like comprising, in 'combinatiom a light source, a polarizing ,element positionedto intercept beams emanating from said source, an'analyzer positioned with its polarizing axis substan-' -tially at right angles to the polarizing axis of said polarizing element to intercept 'light emanating from said source and traversing said polarizing element, a' sensitive tint plate comprising a transparent resilient sheet 'having a high photoelastic coefhcient and showing substantially no cold flow positioned between the polarizer and the analyzer, means to impar't t'o said sensitive rection of said Stretch substantially at 45 to the direction of vibration of light transmitted by said polarizing element, and means to so position said polarizing element and said analyzer that objects to be examined may be placed therebetween.

4. In a device for examining photoelastic effects and the like and comprising apolarizer and an analyzer positioned with their polarizing axes at substantially right angles to each other and spaced from each other so as to permit objects to be examined to be placed therebetween, a transparent resilient sheet of vinyl aceto acetate showing substantally no cold flow and having a high photoelastic coeflicient, means to position said sheet between said polarizer and said analyzer, means to stretch said sheet substantially uniformly and in a direction at an angle to the direction of vibration of light transmitted by said polarizing element of substantially 45 5. In a device of the character described, in combination, a polarizer, an analyzer positioned I with its polarizing axis substantially at right angles to the polarizing axis of the polarizer, a resilient sensitive tint plate positioned between the polarizer and the analyzer and comprising a sheet of vinyl aceto acetate plasticized with trglycoldihexoate, and means to impart to said sensitive tint plate in a direction at an angle of substantially 45 to the polarizing axis of said analyzer a substantially uniforrn predetermined stretch, said means comprising means for fixedly holding one edge of the said sensitive tint plate and rotatable means mechanically connected to the opposite edge thereof. g

EDWIN H. LAND. 

